1. Field of the Invention
The invention relates to a circuit for controlling a picture tube in a television receiver.
2. Related Art
An important accordance with the preamble of claim 1. An important quality characteristic of the image reproduction in a television receiver is the maximum brightness which can be represented on the viewing screen, the so-called light at peak white. This light at peak white is limited in practice essentially by two limitations. Firstly, by the control range or the saturation limit of the RGB output stages controlling the picture tube and, secondly, by the maximum permissible drive of the picture tube.
In order to achieve the highest possible brightness, it is therefore necessary to set the drive of the picture tube, that is to say the amplification of the RGB signals controlling the picture tube, as high as possible in the direction of white. Substantial tolerances are active, in this case such as, for example, the cutoff calibration, the transparency of the picture tube glass and fluctuations in the input signal. Because of these tolerances, driving the video output stages to full output more or less reaches the saturation limit of the amplifiers. Furthermore, it has to be borne in mind that both the settings accessible to the customer such as brightness, contrast and colour saturation, and the ageing of the picture tube which is compensated by the dark current regulation can lead to further driving of the output stages.
If, now, one of the output stages driving the picture tube goes into saturation, so-called smears appear on the viewing screen in the corresponding colour in the form of an extension of a pixel or of an image area in the line direction. These smears occur principally for the colour red, because the picture tube requires the highest drive owing to the phosphorous sensitivity of the red gun.
In order to arrive at such saturation phenomena in the case of the said tolerances, it would therefore be necessary to observe a certain safety distance in driving the output amplifier to full output. However, this means in turn that the theoretically maximum possible drive to full output is not reached as a rule.